You have a big decision to make. Whether to put in an offer on a house, say, or change jobs. Which of the following will help you make the right choice: being in a state of sexual excitement or having a full bladder? Most likely, it is not something you have pondered. Psychologists, however, have long studied the ways that external factors such as these influence our decision-making.

A full bladder, apparently, helps us take more rational, long-term decisions. At least that was the finding of a study carried out by Mirjam Tuk, a professor at the University of Twente in the Netherlands. In an experiment, Tuk tested two sets of participants. The first was told to drink five cups of water; the second to take just five sips. After 40 minutes, they were tested on their decision-making abilities. Tuk says that the group who were, by now, crossing their legs, were better able to exercise self-control and hold out for larger rewards.

If true, Tuk’s findings are interesting because they challenge an established psychological theory called “ego depletion”. This states that we only have a finite well of self-control. Each time we deny ourselves something—whether going to the bathroom or choosing a salad for lunch, rather than the sausage sandwich we really wanted—we use up some of our reserves. The theory of ego depletion was developed by Roy Baumeister, a psychologist at Florida State University. In practical terms, he says, this means that if you have an important decision to make, you shouldn’t draw from your well of self-control beforehand. Reserves, he says, can be depleted in all sorts of ways: “Even things like trying to look interested at a boring meeting, trying to pretend your boss’s jokes are funny or not saying something unkind to your spouse when you are angry.”

Decision-making ability can be thought of as akin to a muscle, Baumeister says, in that it is liable to be worn out with overuse. For some, this effect can be life-changing. A recent study by Israeli scientists found that judges were much more likely to grant a convict parole if they had just eaten a meal than if they were at the end of a session. At first, the scientists thought that falling sugar levels might be making the judges grumpy. But in the end they concluded that making too many decisions was tiring the poor things out. So, as a sitting wore on, they were more likely to take the easy option—in this instance returning the prisoner to chokey.

People also react in surprising ways to their physical surroundings. Josh Ackerman, a psychology professor at Massachusetts Institute of Technology, says that a decision can be affected by something as trivial as what we are touching at the time. His research has shown that our choices can depend on whether we are holding something heavy or light. He carried out an experiment in which participants conducted mock job interviews. Sometimes the interviewers held a heavy clipboard, sometimes a light one. When holding a heavy clipboard, the interviewer was more likely to view the applicant as having gravitas. Those interviewed by someone holding a light clipboard were seen as more flaky. It seems our minds take many such physical metaphors literally. Other studies have found that sitting on a hard chair when negotiating the price of a car will make you negotiate harder, and that holding a hot cup of tea when meeting someone will make you more warmly disposed towards them than if you are holding an iced drink.

The reason, Ackerman believes, is that touch is the first sense that we develop after birth. “People learn how to make decisions about the world by understanding what they’ve already experienced, and that means the physical world.” The ability to think in the abstract, of course, comes much later.

Another area of interest to the study of decision-making revolves around sleep deprivation. Professor Michael Chee, a neuroscientist at the National University of Singapore, says that most of us already realise that making decisions when we are exhausted is not advisable. Yet the effect that tiredness has on us is counter-intuitive. Chee says that we tend to believe that we become overly cautious in order to compensate. But the truth, he says, is that a lack of sleep makes us much too optimistic in our decision-making.

Chee’s research has implications for those who make some of the most important decisions of all. He says that he has evidence that surgeons at the end of a long shift can become too gung-ho, attempting risky treatments in the belief that they have found a genuine shortcut. (This is also something long understood by casino operators in Las Vegas. It is one reason why they go to such lengths to keep punters at the tables until the early hours, by pumping oxygen into the gambling halls and having no windows or clocks on the walls.)

To return to the original question, being sexually aroused, alas, is not an aid to decision-making. Studies have shown that it can make us impetuous, much as you’d expect. But what is striking is that we greatly underestimate its effect. When Dan Ariely, a professor at Duke University in America, and author of “Predictably Irrational”, experimented on his undergraduates, he found that they had no idea of the extent to which they were being led by their libidos. One reason, Ariely believes, is that we have only a limited mix of emotion and cognition to draw upon. Increase one and you automatically detract from the other. So Robin Williams may have been making a valid psychological point when he said: “God gave men both a penis and a brain, but unfortunately not enough blood supply to run both at the same time.”

But if all our decisions are so influenced by external factors, that raises an inevitable question: to what extent are we involved in our own decision-making? Professor Ackerman believes the answer is very little: “All of these subtle influences suggest that most of what is causing our behaviour we are really not aware of. People are just very good at post-hoc reasons for their behaviour.” Which means that next time you reach for that sausage sandwich, you can reasonably lay claim to the excuse that you couldn’t help yourself. Unless you are desperate for the loo at the time.

Bill Ridgersis the business-education editor of The Economist and the editor of Gulliver, its business-travel blog